Previously, there is known a fuel-supply system that can supply fuel of different pressures to an internal combustion engine. For example, the patent literature 1 recites a fuel-supply system that includes: a low-pressure pump that pressurizes fuel of a fuel tank to a relatively low pressure and discharges the pressurized fuel; a high-pressure pump that pressurizes the fuel pressurized by the low-pressure pump to a relatively high pressure and discharges the pressurized fuel; a low-pressure fuel injection valve that injects the low-pressure fuel discharged from the low-pressure pump; and a high-pressure fuel injection valve that injects the high-pressure fuel discharged from the high-pressure pump.
In general, in the high-pressure pump, a portion of fuel of a pressurizing chamber leaks from the pressurizing chamber as leaked fuel through a gap between an inner wall of a housing of the high-pressure pump, which slidably contacts a plunger, and an outer wall of the plunger. The leaked fuel, which is the fuel leaked from the pressurizing chamber, is the pressurized fuel that is pressurized to the relatively high pressure. Therefore, when the leaked fuel leaks into a space having a relatively low pressure, the temperature of the leaked fuel becomes high. When this leaked fuel stays in this space for a relatively long period of time, the temperature of the high-pressure pump becomes high. Therefore, the fuel in the gap between the inner wall of the housing and the outer wall of the plunger may possibly be evaporated to form fuel vapor. Normally, smooth slide movement between the housing and the plunger is maintained when the fuel in the gap is in a form of liquid. However, when the fuel in the gap is evaporated to form the fuel vapor, the smooth slide movement between the housing and the plunger becomes difficult. Therefore, seizing may possibly occur between the inner wall of the housing and the outer wall of the plunger to possibly cause a damage of the high-pressure pump.
A method of directly returning the leaked fuel to the fuel tank is conceivable. However, there are some disadvantages, such as an increase in the number of pipes for returning the leaked fuel and a difficulty of providing a space for installing these pipes depending on an install location of the high-pressure pump.
In the fuel-supply system of the patent literature 1, the fuel, which is discharged from the low-pressure pump, is once stored in a low-pressure chamber in the high-pressure pump, and then a portion of this stored fuel in the low-pressure chamber is supplied to the low-pressure fuel injection valve. At this time, due to the reciprocation of the plunger, a portion of the leaked fuel flows into the low-pressure chamber. Therefore, the portion of the leaked fuel can be discharged to the outside of the high-pressure pump. However, all of the leaked fuel cannot be discharged to the outside of the high-pressure pump, so that there is a possibility of that the temperature of the high-pressure pump is increased. Thereby, there is a possibility of that the seizing occurs between the inner wall of the housing and the outer wall of the plunger.
Furthermore, the patent literature 1 recites an embodiment where the fuel discharged from the low-pressure pump is first introduced into the leak chamber, and thereafter a portion of this fuel is supplied to the low-pressure fuel injection valve. In this way, all of the leaked fuel of the leak chamber is discharged to the outside of the high-pressure pump. However, the temperature and the pressure of the fuel supplied to the low-pressure fuel injection valve may be changed by the temperature and the pressure of the leaked fuel. Therefore, there is a possibility of that injection characteristics of the low-pressure fuel injection valve are changed.